Resolve filename values on load

[kwokcb]

Issue

When loading in a document where the input is a filename, the string is not resolved it is possible
that the incorrect path is used. Example cases include:

1. If a fileprefix attribute is set on a any parent. The attribute value should be prepended to the file name
2. If there are any tokens, including UDIM tokens, they will not be resolved.

Fix

Update to use getResolvedValueString() for filename input types.

Example

The "wood" sample which comes with the release of MaterialX does not load the correct images since it uses fileprefix
The following has the correct image loaded after the code change which prepends the "../../images" path.

It would otherwise render "black".

This is the input file:

<?xml version="1.0"?>
<materialx version="1.39" colorspace="lin_rec709">
  <nodegraph name="NG_wood1" fileprefix="../../../Images/">
    <tiledimage name="image_color" type="color3">
      <input name="file" type="filename" value="wood_color.jpg" colorspace="srgb_texture" />
      <input name="uvtiling" type="vector2" value="4.0, 4.0" />
    </tiledimage>
    <tiledimage name="image_roughness" type="float">
      <input name="file" type="filename" value="wood_roughness.jpg" />
      <input name="uvtiling" type="vector2" value="4.0, 4.0" />
    </tiledimage>
    <output name="out_color" type="color3" nodename="image_color" />
    <output name="out_roughness" type="float" nodename="image_roughness" />
  </nodegraph>
  <standard_surface name="SR_wood1" type="surfaceshader">
    <input name="base" type="float" value="1" />
    <input name="base_color" type="color3" nodegraph="NG_wood1" output="out_color" />
    <input name="specular" type="float" value="0.4" />
    <input name="specular_roughness" type="float" nodegraph="NG_wood1" output="out_roughness" />
    <input name="specular_anisotropy" type="float" value="0.5" />
    <input name="coat" type="float" value="0.1" />
    <input name="coat_roughness" type="float" value="0.2" />
    <input name="coat_anisotropy" type="float" value="0.5" />
  </standard_surface>
  <surfacematerial name="Tiled_Wood" type="material">
    <input name="surfaceshader" type="surfaceshader" nodename="SR_wood1" />
  </surfacematerial>
</materialx>